There is quite a large amount of Cerastium tomentosum in the grass at the foot of limestone cliffs on the Great Orme but not any C. arvense which I have noticed. Nevertheless this plant is the hybrid between the two and had a small amount of "tomentosum" characteristic but with flowers which look (to me anyway) very like C. arvense. It also conveniently flowers well before he main crop of C. tomentosum and can be distinguished by time of flowering if you go to see it early enough.